[ARM][ParallelDSP] Relax alias checks

When deciding the safety of generating smlad, we checked for any
writes within the block that may alias with any of the loads that
need to be widened. This is overly conservative because it only
matters when there's a potential aliasing write to a location
accessed by a pair of loads.

Now we check for aliasing writes only once, during setup. If two
loads are found to have an aliasing write between them, we don't add
these loads to LoadPairs. This means that later during the transform,
we can safely widened a pair without worrying about aliasing.

However, to maintain correctness, we also need to change the way that
wide loads are inserted because the order is now important.

The MatchSMLAD method has also been changed, absorbing
MatchReductions and AddMACCandidate to hopefully improve readability.

Differential Revision: https://reviews.llvm.org/D6102

llvm-svn: 360567

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